This invention relates to automatic neutral point detecting systems for a hydraulic pump adapted to drive at least one hydraulic actuator, and, more particularly, it is concerned with an automatic neutral point detecting system for a hydraulic pump equipped with a displacement varying device capable of shifting both in a (+) direction and in a (-) direction, the displacement varying device being automatically operated to regulate the delivery of the pump, to thereby effect control of the operation of the hydraulic actuator.
Proposals have in recent years been made to equip hydraulic pumps used in various types of hydraulic machines and hydraulic apparatus with a displacement varying device which is capable of shifting both in a (+) direction and in a (-) direction and to control such displacement varying device electrically to regulate the delivery of the pump, in order to conserve energy and improve the performance. In this control system, displacement or shifting of the displacement varying device of a hydraulic pump is automatically detected and a signal is produced for regulating the delivery of the hydraulic pump. In this case, it is essential that the neutral point of the hydraulic pump and the signal produced by a device for detecting the shifting of the displacement varying device exactly match. To this end, it has hitherto been usual practice to provide a control system for a hydraulic pump with a zero point compensating circuit for detecting the neutral point of the hydraulic pump and compensating the signal produced by the shifting detecting device.
More specifically, a hydraulic pump provided with a displacement varying device capable of shifting both in a (+) direction and in a (-) direction is connected to at least one hydraulic actuator and forms a hydraulic circuit for driving the hydraulic actuator. The displacement varying device may comprise a swash plate or a bent axis depending on the type of the hydraulic pump. In the description to be set forth hereinafter, the displacement varying device will be described as comprising a swash plate. In this type of hydraulic circuit, the swash plate is driven for operation by a regulator in accordance with an electric signal inputted to the control system. Associated with the swash plate is a shifting detector for detecting the tilting of the swash plate which comprises a potentiometer and the like. The shifting detector is mechanically connected to the swash plate and produces a signal indicative of the shifting of the swash plate. The shifting detector is electrically connected to a zero point compensating circuit for compensating the zero point which comprises a variable resistor and an adding circuit. The principle of the zero point compensation and the structure of the zero point compensating circuit are described later. An operation lever for operating the actuator produces a signal indicative of a manipulated variable. A signal produced by the zero point compensating circuit and the signal produced by the operation lever are supplied to the control system which generates a control signal for effecting control to bring the shifting of the swash plate into agreement with the shifting commanded by the operation lever.
The principle of the zero point compensation and the structure of the zero point compensating circuit will be described in detail. In order for the shifting detector to produce a correct signal indicative of the shifting of the swash plate, the neutral point of the hydraulic pump or the neutral point of the swash plate should coincide with a neutral point signal produced by the shifting detector when the swash plate is in the neutral position, or a signal OV, for example. However, since the swash plate is mechanically connect to the shifting detector, difficulties are experienced in connecting them together in such a manner that a perfect agreement can be reached between the neutral point of the swash plate and the signal (OV) produced by the shifting detector as corresponding to the neutral position of the swash plate. If there is a difference between the neutral point of the swash plate and the signal produced by the shifting detector, then the shifting detector would produce a signal indicating that the swash plate is located in the neutral position in spite of it not being located in the neutral position, and cause the control system to effect control on the premise that the swash plate is located in the neutral position. As a result, the swash plate would not actually be located in the neutral position in spite of the fact that the operation lever is operated to bring the swash plate to the neutral position or it commands the hydraulic pump to reduce its delivery to zero, so that hydraulic fluid would be discharged through one of a pair of ports of the hydraulic pump. When this occurs, the hydraulic fluid thus delivered by the hydraulic pump would flow through a relief valve to a reservoir, thereby wasting the energy of the hydraulic fluid. Moreover, even if the operation lever is manipulated to command the hydraulic pump to deliver the hydraulic fluid in the same volume for both of the pair of ports of the pump, the actual deliveries at the ports would have different valves. Also, even if an attempt is made to move the actuator in one direction, the situation might arise in which the actuator would move in the opposite direction for an instant before moving in the correct direction, thereby adversely affecting the operation of the actuator.
To obviate these problems, it has hitherto been usual practice to provide the control system with a zero point compensating circuit to compensate for any discrepancy that might exist between the neutral point of the swash plate and the signal produced by the shifting detector. More specifically, the zero point compensating circuit operates such that when there is a difference between a signal that should be produced by the shifting detector when the swash plate is in the neutral position and the signal actually produced by the shifting detector, a variable resistor produces a signal indicative of the difference, and the signal is added to the output signal of the shifting detector by the adding circuit to compensate the output signal of the shifting detector. Thus, the neutral position of the swash plate and the signal inputted to the control system can be brought into agreement with each other, thereby obviating the aforesaid disadvantages of the prior art.
The compensation to be commanded by the variable resistor of the zero point compensating circuit is set as follows. The hydraulic pump is driven by a prime mover while the operation lever is brought to a neutral position. The variable resistor is adjusted in such a manner that the hydraulic fluid discharged through the two ports of the hydraulic pump becomes zero in volume or the swash plate shifts to a neutral position while the operation lever is in the neutral position as aforesaid. The value of the variable resistor reached at this time is one which should be used for effecting compensation.
Some disadvantages are associated with the use of the zero point compensating circuit of the prior art. First, it is troublesome to manually effect adjustments of the variable resistor. The trouble increases in proportion to an increase in the number of hydraulic pumps. It would be apparent that it would not be desirable for adjustment of the variable resistor to be handled in the process of production of the hydraulic pumps before they are shipped to the users. Moreover, even if it is assumed that the variable resistors are correctly adjusted before being sold to the users, there would occur drift in the zero point compensating circuit due to changes in temperature, and disagreement due to changes in the connection between the swash plate and the shifting detector with time. The operation which should be performed to remedy them is time-consuming and labor-wasting because constant inspection and asjustment are required.